This invention relates to an electroluminescent device comprising at least one electroluminescent organic layer including a light emitting material layer interposed between an anode and a cathode, wherein the light emitting material layer emits light upon application of a voltage between the anode and the cathode.
The electroluminescent phenomenon of organic material was observed on anthracene single crystals (J. Chem. Phys., 38 (1963), 2042). Thereafter, a relatively intense luminescent phenomenon was observed using a solution electrode having high injection efficiency (Phys. Rev. Lett., 14 (1965) 229). Thereafter, active research works were made on organic luminescent materials between conjugated organic host materials and conjugated organic activators having a fused benzene ring (U.S. Pat. No. 3,172,862, U.S. Pat. No. 8,710,167, J. Chem. Phys., 44 (1966), 2902, Application No.: 09/914,207 and J. Chem. Phys., 50 (1969), 4364). The organic luminescent materials listed herein, however, suffer from the drawbacks of increased film thickness and a high electric field needed to induce luminescence.
As one countermeasure, researches were made on thin-film devices using evaporation technique and succeeded in lowering drive voltage. Such devices, however, failed to provide luminance at a practically acceptable level (Polymer, 24 (1983), 748, and Jpn. J. Appl. Phys., 25 (1986), L773).
Recently, Eastman Kodak proposed a device in which a charge transporting layer and a light emitting layer are formed between electrodes by an evaporation technique, accomplishing a high luminance at a low drive voltage (Appl. Phys. Lett., 51 (1987), 913 and U.S. Pat. No. 4,356,429). Thereafter, research works were further activated, as by shifting to three layer type devices in which carrier transporting and light emitting functions are separated. From then onward, the study on organic electroluminescent devices entered the practical stage (Jpn. J. Appl. Phys., 27 (1988), L269, L713).
However, there remains a serious problem of product lifetime as demonstrated by a luminescent life which is 3,000 hours at the shortest and several ten thousands of hours at the longest when operated at several hundreds of candelas.
It was also found that the above-described devices are prone to delamination due to moisture adsorption and thermal degradation and substantially increase dark spots during long-term service. It is believed that such degradation is mainly caused by interfacial separation between the inorganic electrode and the organic layer and the potential barrier between the electrodes and the respective carrier transporting materials although these problems remain outstanding.
An object of the invention is to provide an organic electroluminescent device in which the drive voltage is lowered in order to prevent degradation thereof.
Making extensive investigations to attain the above object, the inventors have found that in an electroluminescent device comprising at least one electroluminescent organic layer interposed between the anode and the cathode, especially an electroluminescent device in which an organic hole transporting layer and a light emitting material layer are sequentially deposited on an inorganic electrode (ITO electrode etc.) serving as the anode and the cathode is disposed thereon, improved adhesion and an effective lowering of device drive voltage are achieved by providing an auxiliary carrier transporting layer between the anode and the organic layer (especially between the inorganic electrode and the organic hole transporting layer), and forming the auxiliary carrier transporting layer mainly from the aromatic amine derivative of the general formula (1), and especially, the soluble, electrically conductive compound or polymer that the aromatic amine derivative forms with a dopant.
Specifically, the invention provides an electroluminescent device comprising an anode, a cathode, and at least one electroluminescent organic layer interposed therebetween, wherein a luminescent material in the organic layer emits light upon application of a voltage between the anode and the cathode, characterized in that an auxiliary carrier transporting layer which contains an aromatic amine derivative comprising recurring units of the following general formula (1) and having a number average molecular weight of 200 to 100,000, and especially a soluble conductive compound in which the aromatic amine derivative forms a salt with an electron accepting dopant, is formed between the anode and the organic layer. 
Herein R1 to R4 each are independently hydrogen, a hydroxyl group, substituted or unsubstituted monovalent hydrocarbon group, organooxy group, acyl group or sulfonate group, excluding the case where all R1 to R4 are hydrogen atoms at the same time, and n is a positive number of 2 to 3,000.